Varactor structure and method for fabricating the same

Abstract

A varactor structure with high quality factor and good linearity, and a method for fabricating the same are disclosed. According to the method, an additional ion implantation is performed between a first electrode ion implantation and a second electrode ion implantation to form a high doped region. In other words, a high doped region of the same conductive type as the second electrode is disposed between the second electrode and the substrate. The varactor with additional high doped region not only has a high quality factor and good linearity, but also a high tuning ratio.

1 . A method for fabricating a varactor structure, comprising: 
 (a) providing a substrate having an ion well of a first conductive type, and a plurality of isolation structures disposed around the ion well of the first conductive type;    (b) forming a gate structure on the substrate upon the ion well of the first conductive type;    (c) performing an ion implantation of a first concentration on the surface of the substrate to form at least one high doped region of the first conductive type in the ion well of the first conductive type; and    (d) performing an ion implantation of a second concentration to form at least one electrode doped region of the first conductive type in the high doped region of the first conductive type.    
   
   
       2 . The method of  claim 1 , wherein the substrate is a substrate of a second conductive type.  
   
   
       3 . The method of  claim 1 , wherein the substrate further comprises a deep ion well of a second conductive type disposed in the substrate and around the ion well of the first conductive type.  
   
   
       4 . The method of  claim 1  further comprising performing an ion implantation of a third concentration on the substrate after step (b) to form at least one light doped region of the first conductive type in the ion well of the first conductive type.  
   
   
       5 . The method of  claim 4 , wherein the third concentration is lower than the first concentration and is lower than the second concentration.  
   
   
       6 . The method of  claim 1  further comprising forming a spacer structure outside the gate structure after step (c).  
   
   
       7 . The method of  claim 1  further comprising forming a spacer structure outside the gate structure after step (b), wherein step (c) is a tilt ion implantation.  
   
   
       8 . The method of  claim 1  further comprising a thermal process.  
   
   
       9 . A varactor structure, comprising: 
 a substrate,    an ion well of a first conductive type disposed in the substrate;    a plurality of isolation structures disposed in the substrate around the ion well of the first conductive type;    a gate structure disposed on the surface of the substrate and upon the ion well of the first conductive type;    two high doped regions of the first conductive type disposed in the ion well of the first conductive type under both sides of the gate structure respectively; and two electrode doped regions of the first conductive type disposed in the high doped regions respectively.    
   
   
       10 . The varactor structure of  claim 9 , wherein the distance between two high doped regions of the first conductive type is smaller than the distance between two electrode doped regions.  
   
   
       11 . The varactor structure of  claim 9 , wherein the high doped regions of the first conductive type contact each other in the substrate under the gate structure.  
   
   
       12 . The varactor structure of  claim 9 , wherein the substrate is a substrate of a second conductive type.  
   
   
       13 . The varactor structure of  claim 9 , wherein the substrate further comprises a deep ion well of a second conductive type disposed in the substrate and around the ion well of the first conductive type.  
   
   
       14 . The varactor structure of  claim 9 , further comprising two light doped regions of the first conductive type disposed in the ion well of the first conductive type under both sides of the gate structure respectively, wherein the doping concentration of the light doped region is lower than the doping concentration of the high doped regions of the first conductive type and is lower than the doping concentration of the electrode doped regions of the first conductive type.  
   
   
       15 . The varactor structure of  claim 9  further comprising a spacer structure disposed outside the gate structure.  
   
   
       16 . A varactor structure, comprising: 
 a substrate;    an ion well of a first conductive type in the substrate;    a plurality of isolation structures disposed in the substrate around the ion well of the first conductive type;    a gate structure disposed on the surface of the substrate and upon the ion well of the first conductive type; and    a high doped region of the first conductive type disposed in the ion well of the first conductive type under both sides of the gate structure and directly under the gate structure.    
   
   
       17 . The varactor structure of  claim 16  further comprising two electrode doped regions of the first conductive type disposed in the high doped region under both sides of the gate structure respectively.  
   
   
       18 . The varactor structure of  claim 16 , wherein the substrate is a substrate of a second conductive type.  
   
   
       19 . The varactor structure of  claim 16 , wherein the substrate further comprises a deep ion well of a second conductive type disposed in the substrate and around the ion well of the first conductive type.  
   
   
       20 . The varactor structure of  claim 16 , further comprising two light doped regions of the first conductive type disposed in the ion well of the first conductive type under both sides of the gate structure respectively, wherein the doping concentration of the light doped region is lower than the doping concentration of the high doped regions of the first conductive type and is lower than the doping concentration of the electrode doped regions of the first conductive type.